1. Field of the Invention
The present invention relates to a method for manufacturing a seamed flux-cored welding wire having both a favorable feedability and a low hydrogen content characteristic, the method being excellent in wire drawability. More particularly, it relates to a method for manufacturing a seamed flux-cored welding wire, suitable as an arc welding wire for fully automatic or semi-automatic welding to be used in welding of a mild steel, a high tensile strength steel, a heat-resistant steel, or the like.
2. Description of the Related Art
Arc welding wires for fully automatic or semi-automatic welding include: a solid wire; and a flux cored wire (which is hereinafter simply referred to as a wire or a FCW) formed by filling a flux in a tube-like sheath steel strip (which is hereinafter also referred to as a hoop or a steel hoop). Out of these, FCWs include a type in which the hoop has a junction (which is hereinafter also referred to as a seam), which is an object of the present invention; and a seamless type in which there is no such a junction. The ones of the latter seamless type require a high manufacturing cost, and hence the FCW having a seam is used in a more versatile manner. However, the wording “the state of having a seam” denotes the state of having a gap (opening) with the joint not having been joined by welding or the like, as shown in FIG. 1B described later.
The seamed FCW is generally used for a welding execution process of CO2 gas shield arc welding, MIG welding, or the like. A drawn wire having a small diameter of 0.8 to 1.6 mm is generally used. The seamed FCW is required to be excellent in feedability of the wire during welding, to be low in hydrogen content of the wire, and to be excellent in porosity-resistance during welding as its important product performances.
First, the feedability of the wire will be described by reference to a wire supply device of FIG. 6. The FCW is subjected to welding in such a state as to be wound around a wire spool 30 or loaded in a pail pack. For the use of the FCW in executing welding, the following pushing method is generally adopted. The FCW is drawn from the spool 30 or the pail pack by feed rollers 32 and 33 of the feeder 31. In addition, the FCW is pushed into a liner included in a conduit cable 34 disposed behind. Then, it is fed to a contact tip 40 in a welding torch 37 attached at the tip of the conduit cable 34 through the liner. The FCW is applied with a voltage between the contact tip 40 and a steel material to be welded 38, thereby to carry out arc welding. The wire supply devices include not only the one of the pushing system, but also various ones of a pull system in which there are feed rollers included inside a welding torch, and a wire is drawn through the inside of the conduit cable, and of a push-pull combined system.
The conduit liner herein used is a flexible guide tube formed by shaping a steel wire in a spiral. It generally has a length of about 3 to 6 m, to a long length of 10 to 20 m, which is selectively used according to the distance to the welding site. In such a series of FCW feeding operation, the FCW is required to be fed with stability at a high or a given speed not depending upon the feeding conditions such as the bending angles of the bent portions 35 and 36, and the like, and the feed distance. The wire feeding systems include various ones as described above. However, in any case, the feedability of the wire is one of the important product quality characteristics of the FCW.
Then, when the hydrogen content of the wire is high, a large quantity of porosities due to hydrogen is generated in the welded site, resulting in welding defects. Therefore, as another important product quality characteristic for the FCW, it is required that the hydrogen content of the FCW is low. For the FCW which is excellent in welding bead shape and welding efficiency as compared with the solid wire, the low hydrogen content characteristic is a particularly important quality characteristic in preventing the welding defect.
Such a seamed FCW is generally manufactured in the following manner. With so-called forming steps such as a step of forming a band steel in a U shape, a step of filling a flux in the U-shaped formed band steel, and a step of forming the U-shaped band steel to a tube-like wire, a tube-like formed wire filled inside with the flux is manufactured. Then, the tube-like formed wire is drawn to a product FCW diameter.
Out of these steps, in wire drawing for manufacturing of a seamed FCW, particularly, the substantial wire drawing step part, providing a large processing ratio (reduction ratio), at the initial stage of wire drawing has been predominantly carried out by means of a hole die, (see, e.g., Japanese Unexamined Patent Publication No. 2001-179326, and Japanese Unexamined Patent Publication Nos. 1998-180485, 1998-6083, and 1990-52197). Further, wire drawing lubrication of the substantial wire drawing step part has been predominantly carried out by a wet lubricant such as an animal or plant oil, a mineral oil, or a synthetic oil (see, e.g., Japanese Unexamined Patent Publication No. 2001-179326, Japanese Unexamined Patent Publication No. 1994-15485, and Japanese Unexamined Patent Publication No. 2001-179481). Incidentally, it is also known that, in a method for manufacturing a FCW for stainless steel welding using a stainless steel as a hoop, a roller die is used in place of the hole die, thereby to carry out wire drawing while carrying out intermediate annealing (see, e.g., Japanese Unexamined Patent Publication No. 1999-285892).
However, with such a wire drawing method using a hole die, the shape accuracy of the drawn wire is high. However, the shear force imposed on a lubricant layer at a die surface is large. As a result, a problem of lubricant film breakage tends to occur. Whereas, when the lubrication of the drawing wire is carried out by means of a non-hydrogen-bearing inorganic dry (solid) lubricant causing no problem of the increase in hydrogen, problems of solidification and clogging in the die hole of the lubricant tend to occur.
For this reason, there is a given limitation on the wire drawing speed with a hole die, so that high-speed wire drawing cannot be performed, resulting in a relatively lower wire drawing efficiency. This leads not simply to a problem of productivity but also to a problem of being unable to reduce the hydrogen content of the wire. Namely, when it is difficult to control the moisture content in the atmosphere in a wire drawing step to a trace amount, low wire drawing efficiency and long wire drawing time may also lead to the following problem. Namely, there is a high possibility that the amount of moisture absorbed by the wire (flux) in wire drawing increases to such a degree as to cause welding defects. This problem becomes noticeable not only for a conventional mild steel, but also particularly for the hoop, which requires a larger working force, and imposes a large load on a die, hence facilitates the occurrences of the vibration of the die and the chatter marks of the wire, and is difficult to draw, of an alloy steel, a stainless steel, or the like.
In contrast, when drawing can be carried out by means of a roller die throughout the process or from first to last of the process including the step part at the initial stage of the drawing providing a large working ratio (reduction ratio), no such a problem as to be caused by the hole die occurs. Accordingly, by the use of the non-hydrogen-bearing inorganic dry lubricant, it is possible to increase the wire drawing speed and to reduce the amount of moisture absorbed. This also enables the enhancement of the productivity of the FCWs.
The roller die is, as described later, a wire drawing device for holding a wire in a die hole formed by a pair of opposing roller die components, and carrying out wire drawing. With this configuration, the shear force imposed on the lubricant layer at the die surface is relatively small as compared with the wire drawing using a hole die. Therefore, the problem of the lubricant film breakage is less likely to occur. Further, also when the lubrication for wire drawing is carried out by means of a non-hydrogen-bearing inorganic dry lubricant not causing a problem of the increase in hydrogen content, problems of solidification and clogging of the lubricant as with a hole die does not occur.
However, conventionally, there has been a perception that wire drawing by means of a roller die provides a wire in the shape of not a perfect circle necessary for a FCW, but of an ellipse, resulting in a defective shape accuracy. As a result, the roller die can be used only partially in a wire drawing process. Namely, as is conventionally known, in the part of the step providing a large working ratio at the initial stage of the drawing, and in the final step of the wire drawing, there is no other choice but to use a plurality of hole dies. Further, for the seamed FCWs, including the FCW of a stainless steel hoop, the wire drawing lubrication in the substantial wire drawing step part is predominantly carried out by a wet lubricant such as an animal or plant oil, a mineral oil, or a synthetic oil. However, so long as the wet lubricant is used, for the seamed FCW, even if the reduction in amount of the lubricant to be used, the composition control, and other ideas are implemented, or even however small the seam distance (gap) is, the wet lubricant consequentially enters into the FCW through the seam, resulting in a higher hydrogen content of the wire. In order to prevent this, proper washing of the wire in an off-line process becomes necessary because the wet lubricant cannot be satisfactorily removed by in-line (on-line) simple washing in the wire drawing process.
Whereas, also in the method for drawing the FCW of the stainless steel hoop by means of a roller die, the stainless steel hoop is relatively harder than the mild steel hoop. For this reason, if lubrication is not carried out satisfactorily, the amount of heat generated in the working process is large even with the roller die. Accordingly, the lubricant undergoes film breakage, resulting in a high possibility that the surface roughening of the wire or breakage during wire drawing occurs. Further, annealing at the stage during wire drawing becomes essential due to the work-hardening of the stainless steel hoop. For this reason, there is still a limitation in increasing the wire drawing speed and the wire drawing efficiency for the overall wire drawing steps.
Therefore, under present circumstances, in actuality, a uniform high-speed wire drawing process by means of a roller die, including the step part providing the large working ratio (reduction ratio) at the initial stage of the wire drawing has not been yet put in a practical use in the manufacturing of the FCW. Still, a uniform high-speed and high wire diameter accuracy wire drawing process by means of a roller die, including a step of forming a band steel in a U shape, a step of filling a flux in the U-shaped formed band steel, and a step of forming the U-shaped band steel to a tube-like wire has not been yet put into a practical use.